Environmental location system

ABSTRACT

A system and method for determining a location. The system employs encoded information devices dispersed through the environment, each having, a non-unique code associated therewith. The codes from the encoded information devices are acquired as a reading device passes nearby, and stored. The codes from a proximate set of information devices are correlated with a map or mapping relation to determine one or more consistent positions within the environment. The information devices are preferably passive acoustic wave transponders and the mapping relation may be a pseudorandom sequence or a defined map.

The present application is a continuation of U.S. patent applicationSer. No. 09/902,073, filed Jul. 10, 2001, now U.S. Pat. No. 6,424,916,which is a continuation of U.S. patent application Ser. No. 09/248,023,filed Feb. 10, 1999, now U.S. Pat. No. 6,259,991.

FIELD OF THE INVENTION

A The present invention relates to method and apparatus for determininga location within an environment, and more particularly to a systemwhich derives information from a plurality of passive devices, eachhaving a predetermined location and communicating insufficientinformation to define the predetermined location.

BACKGROUND OF THE INVENTION

A known radio frequency passive acoustic transponder system provides aradio-frequency surface acoustic wave on a piezoelectric substrate whichinteracts with elements on the substrate to produce an individualizedcomplex waveform response to an interrogation signal. The code space forthese devices may be, for example, 2¹⁶ codes, or more, allowing a largenumber of transponders to be produced without code reuse. These devicesconsist of a piezoelectric substrate on which a metallized conductorpattern is formed, for example by a typical microphotolithographyprocess, with a minimum feature size of, for example, one micron, andappropriate antennas and mechanical enclosures. The acoustic wave modeis often a surface acoustic wave (e.g. a Rayleigh wave), althoughacoustic wave devices operating with different wave types are known.

The known transponder devices thus include a surface acoustic wavedevice, in which an identification code is presented as a characteristictime-domain delay pattern in signal retransmitted from the transponder.Typical systems generally require that the signal emitted from anexciting antenna be non-stationary with respect to a signal receivedfrom the tag. This ensures that the reflected signal pattern is easilydistinguished from the emitted signal during the entire duration of theretransmitted signal return, representing a plurality of internal statesof the transponder, allowing analysis of the various delay componentswithin the device.

In such a device, received RF energy is transduced onto a piezoelectricsubstrate as an acoustic wave with a first interdigital electrodesystem, from whence it travels through the substrate, interacting withreflector, delay or resonant/frequency selective elements in the path ofthe acoustic wave, resulting in specific known electro-acousticinteractions. A portion of the acoustic wave energy is ultimatelyreceived an interdigital electrode system and retransmitted. Theretransmitted signal thus represents a complex delay and attenuationpattern function of the emitted signal, and a receiver is provided whichanalyzes the delay and perturbation pattern to characterize the systemwhich produced it: thus identifying the device.

These devices do not require a semiconductor memory nor externalelectrical energy storage system, e.g., battery or capacitor, tooperate. The propagation velocity of an acoustic wave in such a surfaceacoustic wave device is slow as compared to the free space propagationvelocity of a radio wave. Thus, the time for transmission between theradio frequency interrogation system and the transponder is typicallyshort as compared to the acoustic delay intrinsic to the device, so thatan allowable rate of the interrogation frequency change is based on thedelay characteristics within the transponder. The interrogationfrequency is controlled to change by a sufficient amount so that theshortest possible delay path of a return signal may be distinguishedfrom the simultaneous interrogation frequency, and so that all of therelevant delays are unambiguously received for analysis. Further, theinterrogation frequency should not return to the same frequency before amaximum delay period, thus preventing ambiguity or aliasing. Generally,such systems are interrogated with a pulse transmitter or chirpfrequency system.

Systems for interrogating a passive transponder employing acoustic wavedevices, carrying amplitude and/or phase-encoded information aredisclosed in, for example, U.S. Pat. Nos. 4,059,831; 4,484,160;4,604,623; 4,605,929; 4,620,191; 4,623,890; 4,625,207; 4,625,208;4,703,327; 4,724,443; 4,725,841; 4,734,698; 4,737,789; 4,737,790;4,951,057; 5,095,240; and 5,182,570, expressly incorporated herein byreference. Other passive interrogator label systems are disclosed in theU.S. Pat. Nos. 3,273,146; 3,706,094; 3,755,803; and 4,058,217.

In its simplest form, the acoustic transponder systems disclosed inthese patents include a radio frequency transmitter capable oftransmitting RF pulses of electromagnetic energy. These pulses arereceived at the antenna of a passive transponder and applied to apiezoelectric “launch” transducer adapted to convert the electricalenergy received from the antenna into acoustic wave energy in thepiezoelectric material. Upon receipt of an electrical signalcorresponding to the RF interrogation wave, an acoustic wave isgenerated within the piezoelectric material and transmitted along adefined acoustic path. This acoustic wave may be modified along itspath, such as by reflection, attenuation, variable delay (phase shift),and interaction with other transducers or resonators.

When an acoustic wave pulse is reconverted into an electrical signal, itis supplied to an antenna on the transponder and transmitted as RFelectromagnetic energy. The signal may be reflected back along itsincident path, and thus a single antenna and transducer may be provided,for both receiving and emitting Radio Frequency energy. This energy isreceived at a receiver and decoder, typically at or near the samelocation as the interrogating transmitter, and the information containedin this response to an interrogation signal is decoded. Designs areknown, with unitary and separate receiving and transmitting antennas,which may be at the same frequency or harmonically related, and havingthe same or different polarization.

In systems of this general type, the information code associated withand which identifies the passive transponder is built into thetransponder at the time that the metallization pattern is finallydefined on the substrate of piezoelectric material. This metallizationalso typically defines the antenna coupling, launch transducers,acoustic pathways and information code elements, e.g., reflectors. Thus,the information code in this case is non-volatile and permanent. Theinformation is present in the return signal as a set of characteristicperturbations of the interrogation signal, such as a specific complexdelay pattern and attenuation characteristics. In the case of atransponder tag having launch transducers and a variable pattern ofreflective elements, the number of possible codes is N×2^(M) where N isthe number of acoustic waves launched by the transducers (pathmultiplicity) and M is the number of reflective element positions foreach transducer (codespace complexity). Thus, with four launchtransducers each emitting two acoustic waves (forward and backward)(N=8), and a potential set of eight (M=8) variable reflective elementsin each acoustic path, the number of differently coded transducers is2048. Therefore, for a largenumber of potential codes, it is necessaryto provide a large number of launch transducers and/or a large number ofreflective elements. However, efficiency is lost with increasingcomplexity, and a large number of distinct acoustic waves reduces thesignal strength of the signal encoding the information in each.Therefore, the transponder design is a tradeoff between device codespacecomplexity and efficiency.

Typically, the sets of reflective elements in each path form a grouphaving a composite transfer function, while each group, representingdifferent acoustic paths, has a different characteristic timing,allowing the various group responses to be distinguished.

The transponder tag thus typically includes a multiplicity of “signalconditioning elements”, i.e., delay elements, reflectors, and/oramplitude modulators, which are coupled to receive the first signal froma transponder antenna. Each signal conditioning element provides anintermediate signal having a known delay and a known amplitudemodification to the acoustic wave interacting with it. Even where thesignal is split into multiple portions, it is advantageous to reradiatethe signal through a single antenna. Therefore, a single “signalcombining element” coupled to the all of the acoustic waves, which haveinteracted with the signal conditioning elements, is provided forcombining the intermediate signals to produce the radiated transpondersignal. The radiated signal is thus a complex composite of all of thesignal modifications, which may occur within the transponder, of theinterrogation wave.

In known passive acoustic transponder systems, the transponder remainsstatic over time, so that the encoded information is retrieved by asingle interrogation cycle, representing the state of the tag, or moretypically, obtained as an inherent temporal signature of an emittedsignal due to internal time delays. In order to determine a transferfunction of a passive transponder device, the interrogation cycle mayinclude measurements of excitation of the transponder at a number ofdifferent frequencies. This technique allows a frequency domainanalysis, rather than a time domain analysis of an impulse response ofthe transponder. Essentially, the composite response of M signalconditioning elements within the transponder tag are evaluated at atleast M different frequencies, allowing characterization of the group ofelements. Displaced in time from each other, N groups of elements may beanalyzed during the same interrogation sequence.

Typically, the interrogator transmits a first signal having a firstfrequency that successively assumes a plurality of frequency valueswithin a prescribed frequency range. This first frequency may, forexample, be in the range of 905-925 MHz, referred to herein as thenominal 915 MHz band, a frequency band that is commonly available forsuch use. The response of the tag to excitation varies with frequency,due to the fixed time delays and attenuation. In some known systems, theexcitation frequency changes over time, so that the retransmittedresponse, due to the acoustic propagation delay of the tag, is at adifferent frequency than the simultaneously emitted signal, thusproviding a retransmitted signal removed slightly from the emittedsignal, so that when cross-modulated, the resulting signal is nearbaseband, but not DC.

Preferably, the passive acoustic wave transponder tag includes at leastone element having predetermined characteristics, which assist insynchronizing the receiver and allows for temperature compensation ofthe system. As the temperature changes, the piezoelectric substrate mayexpand and contract, altering the characteristic delays and otherparameters of the tag. Variations in the transponder response due tochanges in temperature thus result, in part, from the thermal expansionof the substrate material. Although propagation distances are small, anincrease in temperature of only 20° C. can produce an increase inpropagation time by the period of one entire cycle at a transponderfrequency of about 915 MHz: correspondingly, a change of about 1° C.results in a relative phase change of about 18°. The potential range ofvariation in an uncontrolled environment therefore requires an internaltemperature reference/compensation mechanism.

This known sequential frequency excitation (chirp) interrogation surfaceacoustic wave transponder system provides a number of advantages,including high signal-to-noise performance, and the fact that the outputof the signal mixer at the interrogator receiver—namely, the signalwhich contains the difference frequencies of the interrogating chirpsignal and the transponder reply signal—may be transmitted overinexpensive, shielded, twisted-pair wires because these frequencies are,for example, typically in the audio range. Furthermore, since the audiosignal is not greatly attenuated or dispersed when transmitted over longdistances, the signal processor may be located at a position quiteremote from the signal mixer, or provided as a central processing sitefor multiple interrogator antennae.

Passive transponder encoding schemes include selective modification ofinterrogation signal transfer function H(s) and delay functions f(z).These functions therefore typically generate a return signal in the sameband as the interrogation signal. Since the return signal is typicallymixed with the interrogation signal, the difference between the two willgenerally define the information signal for analysis, along withpossible interference and noise. By controlling the rate of change ofthe interrogation signal frequency with respect to a maximum round trippropagation delay, including internal delay, as well as possible Dopplershift, the maximum bandwidth of the demodulated signal may becontrolled. Thus, the known systems employ a chirp interrogationwaveform which allows a relatively simple processing of limitedbandwidth transponder signals.

Known surface acoustic wave passive interrogator label systems, asdescribed, for example, in U.S. Pat. Nos. 4,734,698; 4,737,790;4,703,327; and 4,951,057, include an interrogator comprising a voltagecontrolled oscillator which produces a first signal at a radio frequencydetermined by a control voltage supplied by a control unit. This signalis amplified by a power amplifier and applied to an antenna fortransmission to a transponder. The voltage controlled oscillator may bereplaced with other oscillator types.

For example, as shown in FIG. 2, the signal S1 is received at theantenna 24 of the transponder 20 and split into a number of subsignalsIN by combiner 42. The subsignals are each subject to a different signalmodification element A_(N)(f), T_(N)(f) 40, and returned to the combiner42. Each signal modification element 40 converts a portion of the first(interrogation) signal S1 into a second (reply) signal S2, encoded withan information pattern. The signal conditioning elements 40 areselectively provided to impart a different response code for differenttransponders, and which may involve separate intermediate signals I₀, I₁. . . I_(N) within the transponder. Each signal conditioning element 40comprises a known delay T_(i) and a known amplitude modification A_(i)(either attenuation or amplification). The respective delay T_(i) andamplitude modification A_(i) may be functions of the frequency of thereceived signal S1, constant independent of frequency, or have differingdependency on frequency. The order of the delay and amplitudemodification elements may be reversed; that is, the amplitudemodification elements A_(i) may precede the delay elements T_(i).Amplitude modification A_(i) can also occur within the path T_(i). Themodified signals are combined in combining element 42 which combinesthese intermediate signals (e.g., by addition, multiplication or thelike) to form the reply signal S2 and the combined signal emitted by theantenna 18.

The information pattern is thus encoded as a series of elements havingcharacteristic delay periods T₀ and ΔT₁, ΔT₂, . . . ΔT_(N). Two commontypes of encoding systems exist. In a first, the delay periodscorrespond to physical delays in the propagation of the acoustic signal.After passing each successive delay, a portion of the signal I₀, I₁, I₂. . . I_(N) is tapped off and supplied to a summing element. Theresulting signal S2, which is the sum of the intermediate signals I₀ . .. I_(N), is fed back to a transponder tag antenna, which may be the sameor different than the antenna which received the interrogation signal,for transmission to the interrogator/receiver antenna. In a secondsystem, the delay periods correspond to the positions of reflectiveelements, which reflect portions of the acoustic wave back to the launchtransducer, where they are concerted back to an electrical signal andemitted by the transponder tag antenna. The signal is passed either tothe same antenna or to a different antenna for transmission back to theinterrogator/receiver apparatus. The second signal carries encodedinformation which, at a minimum, serves to identify the particulartransponder.

The modified transponder (second) signal is picked up by antenna 56, asshown in FIG. 7. Both this second signal and the first signal (orrespective signals derived from these two signals) are applied to amixer 68 (four quadrant multiplier) to produce a third signal S3containing frequencies which include both the sums and the differencesof the frequencies contained in the signals S1 and S2. The signal S3 ispassed to a signal processor 102 which determines the amplitude a_(i)and the respective phase φ_(i) of each frequency component f_(i) among aset of frequency components (f₀, f₁, f₂ . . . ) in the signal S3. Eachphase φ_(i) is determined with respect to the phase φ₀=0 of the lowestfrequency component f₀. The signal S2 may be intermittently supplied tothe mixer by means of a switch (not shown), and indeed the signalprocessor may be time-division multiplexed to handle a plurality of S2signals from different antennas 56.

The information determined by the signal processor 102 is passed to acomputer system comprising, among other elements, a random access memory(RAM) 104 and a microprocessor 106. This computer system analyzes thefrequency, amplitude and phase information of the demodulated signal andmakes decisions based upon this information. For example, the computersystem may determine the identification number of the interrogatedtransponder 20. This I.D. number and/or other decoded information ismade available at an output 107 to host computer 108.

In one known interrogation system embodiment, the voltage controlledoscillator 72 is controlled to produce a sinusoidal RF signal with afrequency that is swept in 128 equal discrete steps from 905 MHz to 925MHz. Each frequency step is maintained for a period of 125 microsecondsso that the entire frequency sweep is carried out in 16 milliseconds.Thereafter, the frequency is dropped back to 905 MHz in a relaxationperiod of 0.67 milliseconds. The stepwise frequency sweep 46 shown inFIG. 3B thus approximates the linear sweep 44 shown in FIG. 3A.

Assuming that the stepwise frequency sweep 44 approximates an average,linear frequency sweep or “chirp” 47, FIG. 3B illustrates how thetransponder 20, with its known, discrete time delays T₀, T₁ . . . T_(N),produces the second (reply) signal S2 with distinct differences infrequency from the first (interrogation) signal S1. Assuming around-trip, radiation transmission time of to, the total round-triptimes between the moment of transmission of the first signal and themoments of reply of the second signal will be t₀+T₀, t₀+T₁, . . .t₀+T_(N), for the delays T_(0N), T . . . T₁, respectively. Consideringonly the transponder delay T_(N), at the time t_(R), when the second(reply) signal is received at the antenna 56, the frequency 48 of thissecond signal will be Δf_(N) less than the instantaneous frequency 47 ofthe first signal S1 transmitted by the antenna 56. Thus, if the firstand second signals are mixed or “homodyned”, this frequency differenceΔf_(N) will appear in the third signal S3 as a beat frequency.Understandably, other beat frequencies will also result from the otherdelayed frequency spectra 49 resulting from the time delays T₀, T₁ . . .T_(N−1). Thus, in the case of a “chirp” waveform, the difference betweenthe emitted and received waveform will generally be constant.

In mathematical terms, we assume that the phase of a transmittedinterrogation signal is φ=2πfτ, where τ is the round-trip transmissiontime delay. For a ramped frequency df/dt or f, we have: 2πfτ=dφ/dt=ω. ω,the beat frequency, is thus determined by τ for a given ramped frequencyor chirp f. In this case, the signal S3 may be analyzed by determining afrequency content of the S3 signal, for example by applying it tosixteen bandpass filters (of any implementation), each tuned to adifferent frequency, f₀, f₁ . . . f_(E), f_(F). The signal processor 102determines the amplitude and phase of the signals that pass throughthese respective filters. These amplitudes and phases contain the codeor “signature” of the particular signal transformer 40 of theinterrogated transponder 20. This signature may be analyzed and decodedin known manner.

The ranges of amplitudes which are expected in the individual componentsof the second signal S2 associated with the respective pathways or tapdelays 0-F may be predicted. The greatest signal amplitudes will bereceived from pathways having reflectors in their front rows; namely,pathways 0, 1, 4, 5, 8, 9, C and D. The signals received from thepathways having reflectors in their back rows are somewhat attenuateddue to reflections and interference by the front row reflectors. If anyone of the amplitudes a_(i) at one of the sixteen frequencies f_(i) inthe third signal S3 falls outside its prescribed or predicted range, asshown in FIG. 5, the decoded identification number for that transponderis rejected.

As indicated above, acoustic transponders are susceptible to so-called“manufacturing” variations, due to intertransponder differences, as wellas temperature variations in response due to variations in ambienttemperature. This is particularly the case where small differences intap delays, on the order of one SAW cycle period, are measured todetermine the encoded transponder identification number. Thesemanufacturing and/or temperature variations can, in this case, be in thesame order of magnitude as the encoded informational signal.

As explained above, the transponder identification number contained inthe second (reply) signal is determined, for example, by the presence orabsence of delay pads in the respective SAW pathways. These delay padsmake a slight adjustment to the propagation time in each pathway,thereby determining the phase of the surface acoustic wave at theinstant of its reconversion into electrical energy at the end of itspathway. Accordingly, a fixed code (phase) is imparted to at least twopathways in the SAW device, and the propagation times for these pathwaysare used as a standard for the propagation times of all other pathways.Likewise, in a reflector-based acoustic device, a reflector may beprovided at a predetermined location to produce a reference signal.

The entire process of compensation is illustrated in the flow chart ofFIG. 6. As is indicated there, the first step is to calculate theamplitude a_(i) and phase 9 for each audio frequency f_(i) (block 180).Thereafter, the sixteen amplitudes are compared against their acceptablelimits (block 182). These limits may be different for each amplitude. Ifone or more amplitudes fall outside the acceptable limits, thetransponder reading is immediately rejected. If the amplitudes areacceptable, the phase differences φ_(ij) are calculated (block 184) andthe temperature compensation calculation is performed to determine thebest value for ΔT (block 186). Thereafter, the offset compensationcalculation is performed (block 188) and the phases for the pathways 2,3, 6, 7, A, B and E are adjusted. Finally, an attempt is made to placeeach of the pre-encoded phases into one of the four phase bins (block190). If all such phases fall within a bin, the transponderidentification number is determined; if not, the transponder reading isrejected.

There are a number of other passive remotely readable informationbearing devices, such as bar codes, color codes, other types of radiofrequency devices, and the like.

Known wireless communications systems include various cellular standards(IS-41. IS-95, IS-136, etc.) as well as so-called PCS standards anddata-only standards, including Cellular Packet Data Protocol (CPDP). TheMetricom “Ricochet” system provides a frequency hopping 915 MHz spreadspectrum wireless local data access system. These communicationsstandards, due to their extensive infrastructure, allow a large numberof simultaneous users to communicate over separate communicationschannels within a relatively small band without substantial mutualinterference. Therefore, communications channels may be appropriated fornear real time communications needs, such as voice and navigationaldata.

SUMMARY AND OBJECTS OF THE INVENTION

According to the present invention, a plurality of passive remotelyinterrogable information devices are provided dispersed through anenvironment. A stored or synthesized map relates a set of identificationcodes of proximate information devices with a specific location withinthe environment. The information devices do not necessarily each havesufficient information storage (or transmission) capacity to uniquelydefine a location. However, the information contained in a proximategroup of information devices together carry sufficient information. Inobtaining the information contained in the group of information devices,this may be obtained simultaneously, but preferably it is obtainedsequentially, with a record kept of the relative positions of eachinformation device. Thus, the set of locations and information contentsare used to search a more global map or mapping function to determine anabsolute location.

Where the environment includes a set of predefined paths, e.g., roads orisles, the map may be a set of topologically interconnected onedimensional strings of code sequences. Where the locations are notlimited by predefined paths, and the receiver is free to roam, the mapincludes a two-dimensional array of codes.

The information devices may be randomly dispersed, and thus, thesequence of codes is random, such that it is unlikely that a number ofdevices, e.g., 5 sequential information devices along any path, would berepeated along any other path, and less likely that 10 sequential alongany path would be repeated. Thus, for limited environments, informationcodes from, e.g., 5 or 10 sequential devices would uniquely define alocation of the interrogator. Once a global location within theenvironment is determined, incremental movements within the environmentare more easily tracked, so that often only a single additionalinformation device must be read in order to determine the change inlocation, within the granularity of the spacing of information devices.Thus, relatively simple information devices and receiver devices may beused to accurately define a location.

The information devices are distributed to avoid close proximity ofindistinguishable codes, and to avoid regular or repeating patterns.Thus, the distribution of information devices must be (a) random: (b)pseudorandom, or (c) regular with no repetition along any path alone anypredefined path or two-dimensional surface. Thus, when distributing theencoded information devices, a sensor may be used to read an informationdevice or tag before placement, seeking to ensure that it meets therequirements for efficient localization within the environment. Thesensor may thus “veto” a selection of device which raises a probabilityof ambiguity. A predetermined mapping or mapping function may also bedefined, thus specifying which information device codes are to bepresent at each location.

During arrangement and distribution of the information devices,preferably these are stored in bins or identified. It is thereforeadvantageous to provide a limited number of codes, for example less that256 codes, and more preferably between 16 and 64 different codes.

Where the array of information devices is small, random placements maybe effective. However, where the array is large, it may be advantageousto define a pseudorandom pattern of information devices throughout theenvironment with a pattern which does not repeat over the encompassedarea, or which provides other positional cues to resolve an ambiguitydue to repeated sequences. These pseudorandom sequences may be generatedby relatively simple electronic devices, and-used to control or suggestplacement of devices. The advantage of a pseudorandom placement definedby a mathematical function is that the mapping function is defined bythe compact mathematical function and therefore allows a relatively lowmemory capacity processor to determine location.

Where an identification code pattern follows a pseudorandom sequence,advantageously a pseudorandom pattern generator-based system may be usedto determine the location by correlating a received sequence withpotential paths through the one-dimensional or two-dimensionalpseudorandom pattern space, as appropriate for the application, untilmatches are found. If additional data reveals an error, furthersearching is conducted until a correct placement is determined. Afterthe position is correctly and unambiguously determined, each additionalinformation device code allows a simple nearest-neighbor search withinthe map to determine the change in location. If ambiguity is detected(two possible locations), other cues may be used to determine location,such as distance (wheel revolution sensor), direction (steeringdirection, compass, inertial sensor), inertial presumptions, and thelike.

While a regular pattern of identification codes may also be used, thistechnique may be less efficient at conveying information, according toknown information theory, unless it gains the pseudorandom presentation,in which case it potentially remains less efficient because it lacks asimple mathematical descriptive function.

The system according to the present invention has a number of advantagesover, and differences with respect to other geopositioning systems, suchas GPS, differential GPS. GLONASS. etc., in that submeter accuracy iseasily obtained, jamming is possible only from nearby systems, it canprovide nearly instantaneous lock-on, is subject to no shadowing fromurban structures, and has low cost. Further, the system according to thepresent invention may be integrated into other systems, providingfurther cost sayings due to common processing elements, input andoutput, power supply and/or packaging. Therefore, the system accordingto the present invention may be used in conjunction with such othersystems to provide a coarse and fine positioning accuracy. Thus, ageopositioning system (e.g., GPS), dead reckoning, inertial guidance, orother type of system may be used to define a coarse position, initialstarting position or consistency check. Further, an initial position maybe input manually. This position may be used as an input into thepositioning system according to the present invention to provide astarting point for a search of the database to find the location of theinterrogator. Thus, where the positional ambiguity is substantial over alarge database, the initial position may be defined to allow usefuloperation without requiring a very large number of transponders to beread or a protracted search and analysis of the database to find aconsistent position. Thus, a commercial GPS system may provide apositioning accuracy of ±100 meters. This GPS-derived position, which isinsufficiently accurate to define a highway lane or exit location, maythen be used to define a coarse position, facilitating initiallocalization using the transponder encoding method. Thereafter, the fineposition and changes in position are tracked using primarily thetransponders, assuming they are closely spaced. If they are not closelyspaced, then another system may be used to define the location, with thetransponder locations used to define differential corrections. In theevent that the various localization systems produce inconsistentlocation information, then an error checking routine may be initiated toidentify the source and effect of the error. Once completed, the systemmay recalibrate according to the defined conditions, or alert the user.

It is noted that, according to the present invention, transponders neednot be evenly or regularly spaced through the environment, and thereforeregions of low density and high density may exist. As stated above, thestrategy for use of low and high density transponder codes may differ.Further, in the case of low density transponder environments, it may bedesired to provide a greater encoding capability per transponder. Thus,in a low density environment, a transponder may completely andunambiguously define its position, while in a high density environment,lower encoding capability transponders may be employed. Likewise, in ahigh transponder density environment, transponders of small and greatencoding capabilities may be interspersed.

Since the environment of operation may be uncontrolled, and the storedmaps and distribution of transponders subject to change and mishap, itis preferred that the system according to the present invention operateaccording to an algorithm which is tolerant of interference, misreads,false reads, and non-correspondences between the stored map andenvironmental distribution of transponders. Thus, an error tolerantsystem is preferably provided. One way to provide such tolerance is toprovide an algorithm which, instead of seeking an exact match between astring of codes, compares the actual code string received from thetransponders with the stored map, to determine a correlation, which isconsidered to indicate a correspondence if it exceeds a certainthreshold. While this may increase the potential degree of ambiguity,this can be compensated by correlating longer strings. Statisticalprocessing of the data may be used to increase confidence in a readingto a transponder code, and processing of the RF signal may be used toidentify and characterize interference. However, damage to, movement of,or replacement of transponders would remain as issues. Such errortolerant processing is preferably used in conjunction with secondarylocalization schemes, as discussed above.

The system according to the present invention may employ an acoustic RFtransponder having a code space of 2³-2⁸ for each device, allowing useof a small device with lower precision than required of a device havinga larger code space. Further, with a small number of codes, therequirement of secondary processing of a received signal to define “loworder” codes is eliminated. Thus, the required electronics are simplerand of lower required precision.

The system according to the present invention relies on a memory, forexample an EEPROM memory, which includes sufficient information, eithera map or a mapping function, to correlate the particular identifyingcode of an information device with its location. A sequence of codes,and preferably their relative sequence or locations, is correlated withthe stored map or expanded mapping equation to determine possiblelocations which meet the received sequence. Its the number of codesreceived increases, the number of possible locations decreases, unlessthere is an ambiguity in the map. Mathematical analysis of potentialmapping functions and static maps may be used to eliminate suchambiguities. In the case of a truly random placement of tags, suchambiguities remain possible. Eventually, any ambiguity will (hopefully)disappear or be small. Further, other cues or presumptions may beapplied to help determine location, such as a presumption of continuityin space, inertia, and models of activity, such as “travel to left ofmarkers”.

In order to accommodate errors in the placement of tags, and/or errorsin reading tags, a fault tolerant design is employed. For example,errors may be due to erroneous placement or replacement, or missing ordefective tags. In this case, a memory “overlay” may be provided tocorrect the system output. The memory may be updated adaptively, asnecessary and/or the system correlated with landmarks at knownlocations. Thus, if a device is randomly replaced with another devicewith a different code, the error would become apparent after traversinga few more markers, and the correction memory updated to reflect thechange. If a tag is erroneously read, the memory overlay may becorrupted, but the location would nevertheless be determined afterreading a few more tags.

Thus, in addition to the mapping system, a statistical process isimplemented to assure stabile operation and location determination evenunder noisy conditions. One way to achieve this is to provide that theenvironment be dotted with a greater number of tags than minimallynecessary for the application, providing redundant information in thescheme, and therefore providing error tolerance.

A number of methods are available for determining a location based on aset of identification codes. In a first embodiment, a memoryarchitecture provides an address space in which the row and columnaddresses have some correspondence to positional coordinates. When afirst identification code is received, the memory is searched and allinstances of the occurrence of that code in the memory are identified.When the second code is received, the memory is searched, with emphasison those occurrences of the second code proximate to the first code.Likewise, when subsequent codes are received, the search is narrowed toclusters containing a path through the sequence of codes. When theposition is unambiguously determined, further relative position changesmay be tracked by restricting the search to locations nearby the lastconfirmed location. In a case where a new identification code fails aconsistency check, i.e., where a change in distance from the lastposition is unreasonable, or other positions of interveningidentification codes are skipped, the update of position may besuppressed until further position information is received, confirming orrefuting a putative position. The output position in this case mayrepresent an intelligent prediction of the position based on other data.If the inconsistency is persistent, then the memory system may beupdated to reflect the actual circumstances.

In another embodiment, the position information and identification codesare stored in a memory, indexed by identification code. Therefore, withthe identification code as an input, the matching, locations arereturned. The matching locations of a sequence of identification codesare analyzed to determine a probable path, by determining a clusterlocation consistent with the received data, and then determining theconsistent path.

In a third embodiment, pairs of identification codes representingadjacent positions are stored in memory. When two identification codesare received, the pair forms an address, which is retrieved from thememory. The memory, in turn, stores a pointer to a set of consistentlocations. This set will be geometrically smaller than a set of singleidentification code consistent positions. When a new identification codeis received, a new set corresponding to the updated pair is accessed.Based on the old position, a threshold window is determined, and used toscreen the new set. Where multiple positions match the window, referencemay be made to data representing a larger number of old identificationcodes, which are used to further screen the location. In this case, thethreshold window for older data must be enlarged, to account forpossible position changes of the detector. Assuming a randomdistribution of identification codes, after sensing of a number ofidentification codes, the positional determination may becomeunambiguous. After an unambiguous determination of position, subsequentposition may be determined by predicting a path and updating theprediction based on received data.

In the case of a pseudorandom mapping equation, the locations of encodedtags are prescribed by a formula. This formula may then be evaluated toprovide a complete map. The advantage of this system is that completemaps are not necessary. The simplest way to use this system is toevaluate portions of the mapping equation and storing this in a smallmemory buffer. The buffer is then searched to determine a correspondencewith available data. When a high degree of correspondence is determined,a putative location is output. Additional tag code data is evaluated,and if consistent with the buffered portion of the decoded map, thedetermined location is output. On the other hand, if it is inconsistent,the mapping function space is further searched for potential consistentlocations. It is noted that all possible consistent locations may beidentified. It is also noted that the initial search time may beconsiderable, especially with a low-end microprocessor and a largemapping space: however, with relatively small mapping spaces and/orafter initial localization (the initial location may be inputextrinsically), even a low computing power system would be able toquickly update a position.

Other types of data analyses are also possible.

In one system, for example, each information device holds 2^(S) bits ofinformation, i.e., there are 256 different codes. An environment isseeded with information devices every 12 feet. over an area of 1 squaremile. Thus, about 193,000 dots are provided. Assuming a balanced numberof each code, there will be approximately 756 dots of each type.However, where codes are paired, there will be only about 3 similarpairs. Using, one additional code or other information, the position maygenerally be unambiguously determined, thus, under these circumstances,two codes, or a movement distance of about 24 feet, allows a trifoldambiguity, while a movement of about 36 feet allows unambiguouslocalization.

The system according to the present invention may be used, for example,to provide location information for vehicles on a highway, forintelligent warehouses, and other applications. Because the resolutionis limited only by the type of transponder and quality of receiver, itis possible to reasonably obtain resolutions of less than about 0.5meter for RF transponders, and less than about 1 cm for opticaltransponders, even over very large distances. Precision is limitedprimarily by the initial mapping of the locations of the encodeddevices. Thus, the system according to the present invention is usablein many circumstances where radio-location systems, such as GPS are not.In addition, the information device transponders may be made efficientlyand cheaply, due to the limited range and codespace, while the receivercomplexity resides primarily in the ambiguity resolution analysis,easily handled by presently available microprocessors and/or digitalsignal processors, or application specific integrated circuits (ASIC).Thus, while the initial determination of a location based on a set ofrelatively small codes, and optionally a path prediction, may requiresignificant analysis, this analysis is not beyond the capability ofavailable systems.

The analyzer may combine a number of strategies to achieve a mostefficient result; however, the minimum required response time for themost difficult analysis will determine the processing capabilitiesrequired, and thus increased efficiency gained through choosing a beststrategy may not produce a significantly better or noticeably fasterresult. On the other hand, under circumstances where the processor hasexcess processing capacity, advanced statistical analysis, interpolationand consistency checking of data may easily be implemented.

The receiver for a radio frequency transponder system operates asfollows. An interrogation signal is emitted. Which may be a pulse,continuous way, frequency chirp, frequency hopping spread spectrumcarrier, direct sequence spread spectrum carrier, or the like. Theinterrogation signal interacts with a transponder, which modifies theinterrogation signal and returns it to a receiver. The receiver analyzesthe interrogation signal in known manner to determine the informationcode of the nearest transponder. More distant transponders and noise maybe filtered using known techniques. Adjacent interrogation systems maybe distinguished by time or frequency multiplexing techniques, or byspread spectrum techniques. Because of the limited codespace, however,the receiver has relaxed technical requirements as compared to receiversfor larger codespace devices. The determined code, optionally alone withother information which may help define a location, is passed to ananalyzer, which then outputs a position, and optionally direction,velocity, acceleration, etc.

While a pseudorandom sequence of information devices providesefficiencies in storing a map, this method also poses the difficultiesin maintenance of the physical system to correspond to the generatingalgorithm. For example, a missing information device would have to bereplaced with an identical information device, or an exceptiongenerated. Further, great care must be taken when first implementing thesystem to assure a workable algorithm. Of course, in both a map andalgorithm based system, an exception map may be provided, the use of anexception map reduces the advantages of an algorithm, and may grow largein size over time and may slow analysis. A map-based system may beadaptive (alterable), and thus might avoid the need for separatelystored exceptions.

Where the information devices are established as regular fixtures of ahighway, for example, then these may be incorporate as part of aguidance and control system of a vehicle, for example to preventunintended swerving out of a lane, as part of an intelligent cruisecontrol system, and as part of a geographical localization system.

While a preferred embodiment provides a radio frequency transpondersystem, optical systems may also be used, which may be encoded withcolors, binary optical codes, or other optical indicia.

It is therefore an object of the invention to provide a localizationsystem comprising an information device reader, a memory for storingmapping information, a memory for storing sets of proximate informationdevice codes received by the reader, and a search engine for searchingthe stored mapping information for map regions consistent with the setsof proximate information codes.

It is also an object according to the present invention to provide adistributed set of information devices, each device having a non-uniquecode, said codes being distributed pseudorandomly or randomly throughthe environment space.

It is another object of the invention to provide a data storage mediumcontaining a map or mapping function describing codes of a distributedset of information devices and relating an identification of a devicewith a position thereof.

It is a further object of the invention to provide a method fordetermining a location comprising dispersing through an environmentspace a set of encoded information devices, each having a non-uniqueencoding, in a random or pseudorandom pattern: storing a mapping ofcodes for encoded information devices in conjunction with a locationthereof in the environment space; receiving codes from a set ofproximately disposed information devices; and searching the mapping toidentify a location having consistent set of proximate informationdevices.

These and other objects will become apparent from a review of thedetailed description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top view of a known passive surface acoustic wavetransponder device;

FIG. 2 is a block diagram of a transponder device corresponding to FIG.1;

FIGS. 3A and 3B are time diagrams, drawn to different scales, of theradio frequencies contained in the interrogation and reply signals whichinteract with the transponder device according to FIG. 1.

FIG. 4 is a block diagram showing antenna coupling and acoustic wavepaths for a portion of an acoustic wave transponder device according tothe present invention;

FIG. 5 is graph showing tolerance bins for received group information inthe system according to claim 1;

FIG. 6 is a flow diagram showing the order of calculations foridentifying a code carried by an acoustic wave transponder;

FIGS. 7A and 7B are schematic diagrams of a typical acoustic wavetransponder interrogation system;

FIG. 8 is a block diagram of a location determination system in aguidance system of a vehicle according to the present invention;

FIGS. 9A and 9B show identification code distributions for a constrainedpath environment and free roaming environment, respectively; and

FIG. 9C shows a variable density identification code distribution.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the present invention will now be describedwith reference the drawings. Identical elements in the various figuresare, designated with the same reference numerals.

An interrogation system according to the present invention is providedwhich employs a frequency hopping spread spectrum signal having apseudorandom sequence which excites each of a set of approximatelyevenly spaced frequencies once during each repetition. The interrogationsignal occupies a band of approximately 20 MHz centered at 915 MHz. Theband is divided into 128 discrete frequencies, each of which ismaintained for about 125 μS before hopping to a different frequency,which is preferably not an adjacent frequency. The interrogation signalis generated by a digitally controlled oscillator, including a phaselocked loop with voltage controlled amplifier. The sequence is selectedto evenly spread energy through the band, without concentrating the waveenergy in a narrow range for an extended period, thus effectivelyobtaining the advantages of a frequency hopping spread spectrumcommunication system. Such sequences are known in the art, and may begenerated based on a lookup table or pseudorandom sequence generator.

Known transponder devices typically employ 16 degrees of freedom intheir code space, generated in accordance with the embodiment of FIG. 1by four bi-directional transducers, each wave having two sets ofelements to interact with. Thus, the interrogator must resolve the 16degrees of freedom in order to identify the transponder. In order toresolve these degrees of freedom, at least 16 distinct conditions mustbe applied to the transponder, producing a response which allowssolution of the simultaneous equations. Since at least 16 conditions, inthis case different frequencies, are required, the larger availablenumber of available frequencies allows robustness to interference andincreased accuracy.

According to the present invention, pseudo-uniqueness of transpondercodes within the environmental space is not required. Therefore, a muchsimpler transponder implementation is possible. For example, FIG. 4shows a transponder having a single bi-directional acoustic wavetransducer with two groups of three delay pads and a reflector in eachgroup, along each wave path. This allows, in addition to thecompensation pads (which may not be necessary in a simplified system)four degrees of freedom for each of two paths, thus defining a codespaceof 32.

A microprocessor 76 is provided to control the system, generating thecontrol signals for the digitally controlled oscillator, which in theembodiment shown in FIG. 7, includes a Digital to Analog converter 78,low pass filter 80 and voltage controlled oscillator 72.

Since only 4 discrete excitation parameters, of the 128 available, arerequired for an output of the transponder code, the analysis may proceedon an incomplete data set. Further, because of thus flexibility, thefrequency hopping sequence need not repeat or excite each frequency atthe minimum rate, so long as the analyzer is provided with dataidentifying the specific excitation conditions.

The receiver system includes an antenna 56 and amplifier, which receivesa modified interrogation (second) signal S2 from the transponder 20. Insome embodiments, this second signal S2 may be normalized in amplitudeby an automatic gain control or limiter, since the phase relationshipswithin the signal encode useful information relating to the encoding.However, in many instances, the signal also carries useful informationencoded in the amplitude, which would be lost in a limiter. Therefore, aphase-amplitude response analysis of the transponder signal ispreferred. This phase-amplitude response thus encompasses amplitudevariations, phase variations and/or amplitude and phase variations. Themodified interrogation signal S2 is mixed in a demodulator with arepresentation of the interrogation signal S1. The demodulator is adouble balanced mixer 68, operating at up to at least 1 GHz. Therepresentation of the interrogation signal S1 may be the first signal S1itself, as being simultaneously output, a delayed replica of the signal,or an independently generated signal. The purpose of this mixer 68 is toultimately translate the frequency of the signal to baseband, to allowhomodyne detection of the relative phase-amplitude response of theinterrogation signal S1 represented in the transponder signal S2. Wherethe signals S1, S2 are in phase, the output S3 of the mixer 68 ismaximal, and decreases as the respective phases reach quadrature,turning negative as the signals move completely out of phase. Due to thecomposite nature of the transponder signal S2, being the superpositionof the modifications in each acoustic path in the transponder device 20,as each component of the wave is initially received after a frequencyhop, the relative phase will change. After the transient response, dueto the elements 40 within the signal path, has abated, the relativephase will be static until the next hop. The output of the mixer 68 isalso related to the relative amplitude of the transponder signal S2.

An integrator 70, which may be implemented as a two pole R-C low passfilter, having both time-constants of about 10 μS, and a frequencycutoff of about 100 kHz, receives the output of the mixer 68, and thusproduces a filtered output representing the relative phase for eachexcitation frequency. The filter output is sampled by a sample holdamplifier 100 after the transients have abated and the signal hassettled, for example four to five time constants of the filter, e.g.,40-50 μS.

Of course, the filter 70 need not be so simple, and may, for example,include an active filter, digitally controlled integrator having apredetermined integration period, or other type.

The duration of each hop is longer than the longest delay in atransponder as well as the travel delay. Thus, where a maximum delaywithin a transponder is less than about 10 μS, a stationary frequencydwell period is greater than 10 μS; practically, this dwell period maybe much greater than this minimum amount.

In the preferred embodiment, a single frequency is emitted as theinterrogation signal at any time: however, a plurality of suchfrequencies may be emitted simultaneously or, concurrently. In thatcase, the receiver system may include a multichannel decoder forselectively decoding each of the frequencies simultaneously (thus, forexample, employing a plurality of mixers and integrators), or forselectively decoding one of the channels one at a time. If a digitalsignal processor is employed (rather than analog components), theprocessing power of the device will determine how much parallelism maybe implemented.

The resulting low frequency signal S3, from homodyne demodulation of theinterrogation signal with the transponder signal S2 at the samefrequency, produces a signal with an amplitude related to the averagephase-amplitude relation of the signals entering the mixer 68. Thisamplitude is determined, for example every 125 μS (8 kHz), withfrequency hops occurring at this same rate. Because of the differencesin the transponder signal S2 due the fixed nature of internal delays andthe changing interrogation frequency, the phase-amplitude response ateach frequency hop provides a datapoint for analyzing the various delayst_(N) within the transponder 20.

In performing an analysis of the transponder signal S2, a number ofcompensations and corrections may be made. For example, the round tripsignal delay may be normalized, yielding an estimate of distance by atime of arrival technique. Likewise, any Doppler shift in the signal maybe determined and compensated, allowing an indication of relative speed.This later correction produces a relative frequency shift of thetransponder signal S2 with respect to the interrogation signal S1. Thisfrequency: shift, however, is typically of a relatively low frequency,below the 8 kHz frequency hopping rate, and therefore introduces onlysmall errors, which may be compensated in the analysis. Likewise, otherpotential causes for variations from the nominal delay periods of atransponder, including temperature changes, mask variations,manufacturing variations and random variations may also be compensatedin the analysis, in known manner. Since the determined degrees offreedom correspond to delays, the correction scheme is essentially asshown in FIG. 6.

The relative phase-amplitude output from the integrator 70 is digitizedand stored in memory 104. While FIG. 7 shows a separate signal processor102 and microprocessor 106, it should be understood that the respectivefunctions may be integrated in a single device. The delay coefficientsof the transponder 20 are determined, which correspond to the degrees offreedom, and corrections and compensations applied as necessary.Consistency checking may be performed for each data point, based on theredundant information from the larger number of datapoints availablethan are minimally necessary, excluding from analysis those which arelikely to represent artifacts or interference.

The analyzer thus evaluates a set of simultaneous equations relating theintegrated phase-amplitude responses to the characteristic set of signalperturbations of the passive acoustic transponder 20, compensating theevaluated degrees of freedom for predetermined variances, evaluatingeach integrated phase-amplitude response for consistency with a set ofremaining integrated phase-amplitude responses, and producing an outputof the delay coefficients.

According to the present invention, the interrogator system is providedon a mobile platform, such as a vehicle. As shown in FIG. 8, amicroprocessor 200 controls a vehicular guidance system. Themicroprocessor 200 executes a program defined in non-volatile memory208, and temporarily stores information in volatile memory 206. Themicroprocessor receives navigational information from an inertial sensor216, a directional sensor 218, a wheel rotation sensor 220, a GPSsubsystem 222 and a compass 224. The microprocessor 200 also interfaceswith an input/output system 214, providing a human interface andintegration with other electronic systems.

In an alternate embodiment, the database which stores the mappinginformation is remote from the interrogator device. In this case, aradio frequency communications link, for example employing the 900 MHzcommunication band, Ricochet, or using a CPDP protocol in the cellularcommunications band (about 832 MHz), allows the computer associated withthe interrogator to communicate with the database in order to localizeitself. This remote database system allows the mobile processor tomaintain limited processing capabilities, and, in the case of a cellularcommunication systems, allows a coarse localization based on theproximity to the cellular antenna, thus reducing the amount ofprocessing necessary. In fact, even with a database local to the mobilesystem, the identification of cellular antennas may still be used tolocalize the interrogator to reduce ambiguities.

The microprocessor 200 controls the interrogation cycle, for example bycontrolling a digitally synthesized oscillator 232 and an RF switch 238.The oscillator signal S1 is amplified by amplifier 234, and transmittedthrough antenna 240. As the antenna 240 is proximate to a transponderdevice 250 ⁰, 250 ¹, 250 ², 250 ³, the radiated RF signal interacts witha respective transponder antenna 252, and is received, modified andretransmitted as a transponder signal S2 by the passive acoustic wavetransponders 250 ⁰, 250 ¹, 250 ², 250 ³. The transponder signal isreceived by the antenna 240, and by way of RF switch 238, supplied toamplifier 236. The amplified transponder signal S2 is mixed in mixer 230with a representation of the oscillator signal S1, which is, forexample, delayed by delay 254, which may be a surface acoustic wavedelay line, similar to the transponders 250 ⁰, 250 ¹, 250 ², 250 ³ inconstruction. The output of the mixer 230 S3 is provided to ananalog-to-digital converter 204, which has an integral sample holdamplifier, and input to the digital signal processor (DSP) 202. The DSP202 processes the signal to identify the code of the respectivetransponder 250 ⁰, 250 ¹, 250 ², 250 ³. The microprocessor 200 receivesthe code identifying the transponder 250 ⁰, 250 ¹, 250 ², 250 ³ andprocesses it in conjunction with a map database 212 and an exception mapdatabase 210, to determine a location within an environment seeded withthe transponders 250 ⁰, 250 ¹, 250 ², 250 ³. Inconsistencies are used toupdate the exception map database 210, to improve performance onsubsequent visits to the same location having the inconsistency.

FIGS. 9A and 9B show a constrained path and free path environment,respectively, with 4 bit codes. As shown in FIG. 9A, if the interrogatorsystem detects a series of transponder codes of 0, 6, 9, 5, the onlyconsistent location within the environment is represented by A.Likewise, 5, 10, 2 is only consistent with location B. Thus, arelatively small number of transponder codes must be acquired before theposition is localized.

As shown in FIG. 9B, in an open space, the analysis is somewhat morecomplicated. For example, it may be possible to remain distant from atransponder, thus misreading it. However, the gain of system cangenerally be increased so that, in a worst case, at least one signal canalways be read. Directional antennas and timing differences may be usedto distinguish between multiple transponders. To determine a location, acomplete neighbor analysis must be performed, or additional navigationalinformation provided, such as direction (inertial sensor 216,directional sensor 218, compass 224), distance (wheel rotation sensor220), or coarse location (GPS 222).

In navigating through the environment, for example, the sequence 4, 8,0, B defines the location C, in the 5^(th) column, 5^(th) row,commencing in the 3^(rd) column, 3^(rd) row, heading right and thendiagonally down. A search of the map reveals this to be the onlyinstance of the string B, 0, 8, along any axis, and thus three codeswould have been sufficient to localize the interrogator. If all the dataavailable were the sequence 4, C, then an ambiguity would be present,either D1 in the 3^(rd) column, 8^(th) row, or 2^(nd) column, 3^(rd)row. Therefore, the location would be resolved unless the next code wasa 1 (3^(rd) column, 4^(th) row or 2^(nd) column, 7^(th) row), unlessother data was supplied.

If a transponder code is missing or erroneously read, the map processor200 will detect the error after a few more received codes, as an erroror inconsistency, and store an entry in the exception map database 210.Thereafter (until flushed or reset), the map processor 200 will read themap exception database 210 data in preference (but not necessarily tothe exclusion of) the map database 212 data. The map processor 200 mayalternately perform a correlation of the sequence of received codes withthe potential sequences represented in the map database 212 andexception map database 210, to determine a likelihood of identity. Wherethe correlation coefficient exceeds a threshold, which my be static oradaptive, a localization may be inferred.

The transponder system according to the present invention mayadvantageously be employed as part of an intelligent highway system.Therefore, the passive transponder units may be distributed along theroads as highway “dots”. In this case, the transponder units areenvironmentally sealed, and have an internal antenna. An outer portionof the housing has a retroreflective structure, which improvesvisibility. The retroreflective structure may be, for example, a plasticplate having prismatic structures, or a paint having glass beadstherein.

In an intelligent highway system, automobiles are outfitted with aninterrogator and associated processor. Optionally, the navigationalsystem may be integrated with a cellular telephone communicationssystem. This cellular telephone system integration potentially allowspre-localization of the vehicle to within a five mile radius, andtypically much smaller, based on an identification of proximate cellularantennas. The navigational system may also communicate through thecellular telephone link, but this is not necessary. This communicationsnetwork, however, may advantageously be used to communicate the raw datato a remote server, and return the exact location of the interrogator,or to synchronize the various mobile systems to include accurategeographical (localization) data. Thus, a central server with adispersed communications network is provided.

FIG. 9C shows an intersection 303 of two roads 301, 302, with a trafficcutoff 304. The transponders are situated such that those closest 313,314, 319, 318 to the intersection 303 are spaced at closer intervalsthan those further away 312, 315, 316, 320. Superimposed over the mapare a set of grids, representing latitude and longitude as determinableby a GPS system. As can be seen, the accuracy of the GPS is insufficientto localize the receiver sufficient for navigation.

Off of road 301 is a building 309 with parking lot 308, connected byentrance 307. As shown in FIG. 9C, each parking space 311 within theparking lot may be designated by a different transponder 310. Thus, thedensity and arrangement of transponders may vary based on theenvironmental needs and desired positional resolution. In thisembodiment, for example, each transponder may have, for example, an 8bit code. While, within the scope of FIG. 9, there need not be anyambiguity in encoding with such a set of transponders, it is understoodthat the environmental localization space may extend far beyond thelimits of the figure, and therefore ambiguities may occur over largedistances. Therefore, even the relatively coarse GPS positiondetermination 305, 306, may be sufficient to resolve any positionalambiguity. The maximum spacing of the transponders is not limited,except by the need for data. However, where the spacing exceeds thenormal GPS resolution, e.g., 100 meters, then the GPS would likely beseen as the primary positioning system with the transponders and maplocations, used for example, to provide a differential positioncorrection for the GPS. The minimum spacing of transponders is limitedby the selectivity of the interrogation system and availability of otherfine-grained positioning systems. Typically, the spacing of transponderswill be no closer that 10 centimeters, and more typically 2-10 meters.

As shown in FIG. 8, the microprocessor 200 communicates through acommunications system 260 having antenna 262. This communications systemis, for example, a cellular radio device. A cellular base station 272,having antenna 270, communicates with the communications system 60. Thecellular base station 272 is associated with an identification code 274,which may be transmitted. This identification code 274 is therefore alsoassociated with the particular location of the cellular base station272. The cellular base station 272 employs the IS-41 protocol, whichprovides for hand-offs between base stations for moving transceivers andother system administration functions. The cellular base station 272communicates through a network “cloud” 278 with a server 280, havingassociated with it a database 282. Thus, it can be seen that the mappinginformation may be located in the mobile unit associated map database212 or the server 280 associated database 282, or potentially both. Theserver 280 may directly resolve the location of the interrogationantenna 240, or it may download appropriate mapping references tothrough communications system 260 based on the received identification274 of the cellular base station 272.

By analyzing the return signal for Doppler shift and the like, it ispossible to determine the relative velocity of the vehicle and thetransponder, which is typically stationary.

There has thus been shown and described a novel method forinterrogating, a passive acoustic wave transponder with a frequencyhopping interrogation wave, and a method and system for analyzing atransponder signal therefrom. Many changes, modifications, variationsand other uses and applications of the subject invention will, however,become apparent to those skilled in the art after considering thisspecification and the accompanying drawings which disclose preferredembodiments thereof. All such changes, modifications, variations andother uses and applications which do not depart from the spirit andscope of the invention are deemed to be covered by the invention whichis limited only by the claims which follow.

What is claimed is:
 1. A method of determining a location in anenvironment, comprising: (a) sequentially recording a set of respectivenon-unique identifiers for a subset of a set of landmarks, said set oflandmarks having predetermined relative positions said identifiers beingrecorded in conjunction with a positional relationship therebetween; (b)comparing said sequentially recorded identifiers and positionalrelationships with a map of said predetermined relative positions, todetermine one or more most likely corresponding positions; and (c)outputting position information.
 2. The method according to claim 1,further comprising the step of remotely reading a non-unique identifierfrom a landmark.
 3. The method according to claim 1, further comprisingthe step of wirelessly reading a non-unique identifier from a landmark.4. The method according to claim 1, further comprising the steps ofinterrogating a landmark and receiving the non-unique identifier from alandmark in response to the interrogation.
 5. The method according toclaim 1, further comprising the steps of emitting an interrogationsignal and receiving, in response to the interrogation signal, thenon-unique identifier from a landmark.
 6. The method according to claim1, further comprising the steps of interrogating a landmark with a radiofrequency signal, and receiving from the landmark a modified radiofrequency signal.
 7. The method according to claim 1, further comprisingthe steps of interrogating a landmark with a radio frequency signal, andreceiving from the landmark a backscatter radio frequency signalcomprising the identifier.
 8. The method according to claim 1, whereinsaid comparing comprises the steps of assuming a prior position,analyzing most recently received identifiers and comparing these topredetermined identifiers in a zone around the assumed prior position,and determining at least one consistent position.
 9. The methodaccording to claim 8, wherein, if a plurality of positions areconsistent, a most likely position is output.
 10. The method accordingto claim 8, wherein, if a plurality of positions are consistent, furtherlandmark identifiers are obtained until a unique consistent position isdefined.
 11. The method according to claim 1, wherein each of saidlandmarks comprises a surface acoustic wave passive backscatter radiofrequency identification transponder.
 12. The method according to claim1, wherein at least two landmarks have the same identifier.
 13. Themethod according to claim 1, further comprising the step of implementingan error tolerant algorithm for determining a position in the event thatone or more errors of the following types occur: said predeterminedrelative position is altered, the identifier is miscommunicated, and themap is erroneous.
 14. The method according to claim 1, wherein saididentifiers are not algorithmically defined by a relative position. 15.The method according to claim 1, wherein said comparing comprisescomputing a correlation of sequentially recorded identifiers andrelationships with a map of said predetermined relative positions todetermine consistent corresponding positions in fault tolerant manner.16. The method according to claim 1, further comprising the step ofcalculating a potential ambiguity factor for a landmark having aparticular identifier to be placed at a particular relative positionbased, on a predetermined computational criteria.
 17. The methodaccording to claim 1, wherein said corresponding position is determinedwith an accuracy of between about 0.1 to about 100 meters.
 18. Themethod according to claim 1, further comprising the steps of determininga location using a secondary positioning system; and producing acomposite location based on the determined relative position and thedetermined landmark location.
 19. The method according to claim 18,wherein said secondary positioning system comprises a geopositioningsignal receiver.
 20. The method according to claim 1, further comprisingthe step of determining a path traveled with respect to the landmarks.21. A system for determining a location in an environment, comprising:(a) a memory for storing sequential sets of respective non-uniqueidentifiers for a subset of a set of landmarks, said set of landmarkshaving predetermined relative positions, said identifiers being recordedin conjunction with a positional relationship therebetween; (b) aprocessor for comparing said sequentially recorded identifiers andpositional relationships with a map of said predetermined relativepositions, to determine one or more most likely corresponding positions;and (c) an output presenting the determined position information.